The provision of voltage regulators is well known for generators that develop a voltage dependent both on exciter field current and the generator speed and are used in a service in which the speed continually changes, as for example in motor vehicles. In the known regulators the current through the exciter field winding of the generator is controlled by a switching transistor of which the base electrode is generally connected through a Zener diode to a voltage divider that is, in turn, connected across the exciter field voltage or across the battery voltage of the vehicle. If the voltage at the voltage divider exceeds the value at which the Zener diode begins to conduct in the breakdown mode, the switching transistor is blocked and the current through the exciter field winding drops off. If the voltage at the voltage divider drops back below a value that is no longer sufficient for the Zener diode to conduct, the switching transistor is again put into its conducting condition and the exciter field current rises again. In this manner, a two-position regulator is provided that nevertheless only operates within a certain hysteresis and does not regulate continuously.
In order to improve the behavior of the voltage regulator it is known to use the combination of one or more switching transistors and one or more control transistors instead of a single switching transistor. The operation characteristic of a regulating amplifier so provided can be further improved by various forms of feedback.
From British Pat. No. 979,415 a voltage regulator is known in which a control transistor controls a switching transistor. In this case the output of the switching transistor is fed back by means of an RC network to the base of the control transistor. The base of the control transistor is connected in the manner described above, through a Zener diode, with the tap of a voltage divider. If the Zener diode is put into its conducting condition, the feedback from the switching transistor to the control transistor by the RC network has the effect of making the regulating amplifier unstable. Relaxation oscillations then take place in which the regulating amplifier switches over from a condition in which the control transistor conducts and the switching transistor is blocked to another condition, in which the control transistor is blocked and the switching transistor conducts at a frequency that is determined by the RC network. The keying ratio of this relaxation oscillation, i.e. the ratio to the oscillation period of the duration of the condition in which the switching transistor conducts, is determined by the current through the Zener diode. In this manner a continuously operating self-oscillating two-point regulator is produced from the two-point regulator above discussed. Although this method produces a raised regulation speed, particularly at lower generator speeds at which without the described feedback a flickering of the charge control lamp would take place, nevertheless, as a result of the continual switching back and forth, voltage jumps are produced that undesirably widen the switching hysteresis.
In British Pat. No. 1,205,421 another voltage regulator is disclosed in which in addition to the above described feedback through an RC network, another stiff feedback is used. In this case the switching transistor is current-coupled through an emitter resistor and the voltage at the emitter of the switching transistor is fed back through another resistor to the base of the control transistor, which in turn is connected through a Zener diode with the tap of a voltage divider. The feedback from the collector of the switching transistor through an RC network to the base of the control transistor here operates as a delayed positive feedback, while the feedback from the emitter of the switching transistor through a resistor to the base of the control transistor operates as stiff inverse feedback. Stabilization of the regulating amplifier is obtained by the stiff negative feedback, particularly against variations in the amplification factors of the transistors and variations of the operating parameters of the regulator, without, however, regulating out the distributing switching transients.
A voltage regulator was disclosed in German published patent application (OS) No. 1,613,983 in which the voltage divider is connected across the exciter field winding and is provided with an additional tap, to which a capacitor is connected, of which the other terminal is connected to ground or chassis. the first tap of the voltage divider is connected in the usual way through a Zener diode to the control transistor. The collector of the switching transistor, furthermore is connected through a resistor with the first tap of the voltage divider, i.e. the regulating amplifier has a stiff positive feedback. In addition, for compensation of temperature effects two diodes are connected in series with the Zener diode. By the connection of the capacitor between the additional tap of the voltage divider and chassis ground a smoothing of the command voltage of the regulator is obtained. The regulator is thus constituted as a two-point regulator with two discrete switching conditions.